528 D. M, PRESCOTT 



fixed, extracted to remove acid-soluble material, and autoradiographed. 

 Within only i -5 min. after addition to the medium, pH]-cytidine is taken 

 up, converted to the appropriate form and incorporated into nuclear RNA 

 (Fig. 2(a)). After 5 min. the rate of pH]-cytidine incorporation into the 

 nuclear RNA rises. Incorporated activity is not detected in the cytoplasm 

 until about 12 min. ; in contrast, the nucleus is densely labelled (Fig. 2(6)). 

 After 12 min., label accumulates steadily in cytoplasmic RNA but the rate 

 of accuitmJation of radioactivity in the nucleus recedes to a lower value at 

 about 25 or 30 min. At 35 min. the nucleus and cytoplasm are equally 

 labelled (Fig. 2(r)), and at 60 min. the cytoplasm contains more than twice 

 as much label as the nucleus. The nucleus at this time, however, is still 



(60- 



|120- 



40- 



25 30 35 



TlME(min) 



Fig. I . The two curves show the time course of the total amount of [^H]-cytidine 

 incorporated into RNA of the nucleus and cytoplasm of Tetrahymena with the 

 isotope continuously present in the medium. Each point is the mean grain count 

 for autoradiographs for 23 to 26 cells. The range for each point indicates 95% 

 confidence limits. 



more densely labelled. Ribonuclease digestion shows that DNA synthesis 

 contributes very little to this incorporation. L nlabelled deoxycytidine has 

 been added to the medium with the intention of minimizing pH]-cytidine 

 entrance into DNA in all experiments. The time of appearance of tritium 

 in cytoplasmic RNA varies from one experiment to another. In one case it 

 occurred slightly earlier than 13 min. and in another experiment did not 

 begin until 25 min. In the latter experiment, the longer delay is probably 

 related to a short interruption in cell proliferation imposed by transfer of 

 the log phase cells to fresh nutrient medium just before the experiment 

 was begun. 



